I suppose you and I have a different thing in mind: if you're measuring efficiency, then yeah rank shouldn't matter and nor should stun or kill; if the alien is down, you dun good. I was thinking more of a "experience" level, where higher ranking aliens and races would award more points, and so would stuns over kills.
I'm not fond of the efficiency rating simply because it would be so low. Compounded with poor accuracy, I would find most soldiers would have a rating of near zero. I prefer the "experience" level, because that just keeps going up. It'll be like score, but more customized to the individual soldier.
All this said, most data is being tracked to calculate the efficiency rating, so it could be added too. Experienced soldiers can be inefficient after all ;p